The primary chemical method for making pulp from wood involves the digestion of lignin in the wood with sodium sulfide and sodium hydroxide. This is termed the sulfate or kraft process.
Wood pulp produced in the kraft process generally contains 5-8% by weight of residual, modified lignin which gives the pulp a characteristic brown color. To obtain a pulp of very high brightness and brightness stability, the lignin must be removed by certain oxidizing agents commonly referred to as bleaching chemicals. Many bleaching processes exist but almost all begin with the chlorinatior extraction (C-E) stage. There is a loss of cellulosic fibers during the C-E stage. The C-E effluents resulting from treated pulp contain a very large number of organic compounds having a bound chlorine content of 2.5-3.5 kg/ton pulp. Some of these compounds, primarily the chlorinated phenolics, are known to have toxic, mutagenic and carcinogenic effects. (Alberti, B.N. and Klibanov, A.M. [1981] Biotechnology and Bioengineering Symp. 11:373-379). These effluents are highly unsuited for recycling within the pulping system due to their high level of corrosive chlorides. Alternatives to chlorine bleaching have, therefore, long been sought by industry.
Hydrogen peroxide has been shown to deligninify sulfite pulps satisfactorily, but on its own it is a relatively ineffective means of bleaching kraft pulp. When used in sequences with chlorine-containing bleaching agents, however, peroxide contributes significantly to deligninification, pulp brightness and brightness stability.
Oxygen and ozone have been extensively studied for incorporation into the bleaching processes. The major disadvantage of these compounds is their non-specific oxidative attack on cellulosic fibers. Lower pulp yields tend to result and the pulp properties are generally inferior to those obtained with chlorine bleaching sequencing.
Research sponsored by the U.S. Department of Agriculture's (USDA) Forest Products Laboratory has demonstrated that 50-75% of the residual lignin was removed by fungal cultures of Phanerochaete chrysosporium in 6 to 8 days. Longer incubation resulted in greater lignin reductions, but the data were not quantified. During incubation, the pulp became substantially lighter in color. Furthermore, fungal deligninification of kraft pulps resulted in a concomitant reduction in the amount of chlorine required to produce a given level of brightness (Kirk, T. K. and Chang, H. [1981] Enzyme Microb. Technol. 3:189-196).
Bleaching is impractically slow using whole fungal cultures. It was found that lignin removal (i.e., kappa number decrease) from kraft pulp followed a triphasic pattern: 1) no lignin removal during establishment the fungus in the pulp over the first two days, 2) rapid deligninification during the following two days, and 3) slower deligninification thereafter. The initial two-day lag is due to the secondary metabolic importance of lignin degradation to fungal growth.
Another disadvantage of fungal bleaching is that these organisms contain enzymes which degrade both cellulose and hemicellulose. In any effective bleaching scheme, the degradation of cellulosic fibers must be completely suppressed, since the cellulosic fibers are particularly vulnerable after kraft pulping. Cellulase-less mutants have to some extent overcome this problem, but they are difficult to manage and are even less efficient in degrading lignin than normal fungal cultures. A final disadvantage of using fungal cells is that they can only operate optimally in an environment where temperature and microbial contamination are carefully controlled.